THEORY OF MILK SECRETION 



175 



which it is manufactured will now be discussed. One 

 fact which accelerated this idea was that colostrum 

 was found to contain certain minute bodies called 

 "colostrum corpuscles." Under the microscope, these 

 exhibited traces of cell structure apparently like shed 

 (epithelial) cells from the alveoli. This suggested that 

 milk may be a product formed by the decomposition 

 of the cell walls of the alveoli. 



These alveoli have very thin walls, consisting 

 of a single layer of minute living cells, which 

 are in fact the secreting cells of the mammary 

 gland. If a section were cut so as to remove the 

 tops of the lobules of an alveolus, it would have 

 approximately the following appearance under the 

 microscope : 



Capillary blood-vessels. 

 Epithelial cells of alveolus. 



Epithelial cells breaking up; the cell itself 

 apparently disappears in normal milk. 



Fat-globules among the other secretions which 

 constitute milk. 



The method of secretion can now be fairly easily 

 followed. The capillary blood-vessels which form a 

 network round these alveoli supply the glands of the 

 udder (mammary glands) with blood which is con- 

 stantly circulating through the udder. Part of this 

 is absorbed by the thin walls (epithelial cells) of 

 the alveoli, and these epithelial cells commence to 

 grow towards the inside of the alveolus, but as fast as 



